In the study of bulk semiconductor materials and low-dimensional semiconductor materials, it is very desirable to be able to accurately measure physical characteristics such as energy levels, critical points, electric-field distribution dopant concentration, etc. These characteristics may be measured using reflectance spectrometry which is a contactless and nondestructive measuring system that can operate at room temperature. The measurement of these characteristics is important for evaluating the quality and homogeneity of semiconductor materials.
The use of modulation reflectance spectrometry as a measuring system is known and is discussed at length in U.S. Pat. Nos. 5,287,169; 5,260,772; 5,255,071; 5,255,070; and 5,159,410, all to Pollak. The contents of those patents are incorporated herein by reference.
The technique of modulation reflectance spectrometry measures the effect that a periodically modulated source has on the reflectance spectra of a given sample, instead of directly measuring a given characteristic of the sample.
For instance, photoreflectance (PR) spectrometry uses a modulated light source. Electroreflectance (ER) spectrometry uses a modulated electric field. Other modulation methods can be used such as piezomodulation, thermal modulation, wavelength modulation, etc.
In the case of PR, a modulated light source and a monochromatic light source are directed at a sample through the use of lenses or mirrors. The modulated light source affects various parts of the sample differently depending on specific transitions in the sample composition and structure. The reflectance spectra of the sample is determined by the reflectance of the monochromatic light. The reflected light is received by a detector such as a photodiode, photoconductor, etc. The detector produces signals that are used to measure the desired characteristic and to adjust a variable neutral density filter. The neutral density filter regulates the intensity of the monochromatic light source for normalization.
In the case of ER, the sample is placed in a condenser-like system comprised of a pair of electrodes that produce a modulated electric field in response to an electromodulation source. As with the above noted PR system, a monochromatic light source is directed at the sample and the reflected light is received by a detector which produces signals that are used to measure the desired sample characteristic and to normalize the monochromatic light source.
The problem in the past has been that these systems could only operate on sample sizes with an area of more than 1.0 mm.sup.2. This was due to a limitation in the prior art reflectance systems which could not collect the reflected light of a focused optical spot that was smaller than 1.0 mm.sup.2. This corresponds to an area which is considerably larger than the discrete segments (e.g., gate junctions) that comprise semiconductor devices and prevents the prior art reflectance systems from being used to characterize small-size semiconductor devices and microstructures. This characterization is very important in the analysis of semiconductor materials.
Oftentimes, the purpose of analyzing semiconductor materials is to ascertain the qualities of the various sample areas. These qualities are determined by the various conditions under which a given semiconductor material is grown. Imperfections in the semiconductor material are indicative of improper growth conditions (e.g., temperature, vacuum, growth material, etc.) and to correct these imperfections it is necessary to be able to isolate specifically what step in the process of fabrication caused the imperfections. To do this, one must be able to measure specific microstructures within the semiconductor material.
A further problem with the prior art reflectance system is that it is very difficult to set up the monochromatic light source and the detector to measure the characteristics of a specific desired portion of the sample such as a specific gate junction.
Therefore, it is an object of the present invention to utilize a method of micro-reflectance spectrometry that is capable of measuring sample sizes and sample areas smaller than 1.0 mm.sup.2.
Another object of this invention is to provide an improved apparatus that utilizes micro-reflectance spectrometry to characterize sample sizes smaller than 1.0 mm.sup.2.
A further object of this invention is to provide a method of directing a probing light spot through the use of a microscope capable of achieving a spatial resolution of less than 1.0 .mu.m utilizing micro-reflectance spectrometry.
A still further object of this invention is to provide an improved apparatus that utilizes micro-reflectance spectrometry and is capable of achieving a spatial resolution of less than 1.0 .mu.m.